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An Experimental Study of TLS Forward Secrecy Deployments

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An Experimental Study of TLS Forward Secrecy Deployments An Experimental Study of TLS Forward Secrecy Deployments Lin-Shung Huang∗, Shrikant Adhikarlay, Dan Bonehz, Collin Jackson∗ ∗Carnegie Mellon University, flinshung.huang, [email protected] yMicrosoft, [email protected] zStanford University, [email protected] Abstract—Forward secrecy guarantees that eavesdroppers sim- In response, some sites like Google have deployed forward ply cannot reveal secret data of past communications. While secrecy to protect the privacy of past encrypted communi- many TLS servers have deployed the ephemeral Diffie-Hellman cations [3], using TLS’s ephemeral Diffie-Hellman (DHE) (DHE) key exchange to support forward secrecy, most sites use weak DH parameters resulting in a false sense of security. In or ephemeral Elliptic Curve Diffie-Hellman (ECDHE) key our study, we surveyed a total of 473,802 TLS servers and exchange methods. With these methods, the server’s long- found that 82.9% of the DHE-enabled servers were using weak term secret key is used to sign a short-lived (ephemeral) DH parameters. Furthermore, given current parameter and Diffie-Hellman key exchange message. The resulting Diffie- algorithm choices, we show that the traditional performance Hellman secret is used as the session’s pre-master secret. argument against forward secrecy is no longer true. We compared the server throughput of various TLS setups, and measured Once the pre-master secret is discarded after the session, real-world client-side latencies using an ad network. Our results the session key cannot be reconstructed even if the server’s indicate that forward secrecy is no harder, and can even be faster private key is given. Unfortunately, we discovered in our study using elliptic curve cryptography (ECC), than no forward secrecy. that 82.9% of web sites who support DHE are using DH We suggest that sites should migrate to ECC-based forward parameters weaker than their signature key strengths, resulting secrecy for both security and performance reasons. in a weakened pre-master secret. This suggests an industry- Keywords-TLS; forward secrecy; elliptic curve cryptography; wide misunderstanding of how DHE should be deployed. Our results. In this study, we performed a survey of TLS for- I. INTRODUCTION ward secrecy deployment across the top one million websites. The Transport Layer Security (TLS) protocol is designed to We then evaluate the performance costs of forward secrecy provide privacy and data integrity between two communicating on servers and real-world clients. Some of our results are as parties. TLS is increasingly used on the Internet to protect follows: users’ emails, credit card transactions and other personal data. We successfully scanned 473,802 websites that use TLS. We briefly describe how TLS protects data from eavesdroppers Over 74% of those sites supported at least one of the two as follows. When a client connects to a TLS server, the client ephemeral key exchange methods (DHE or ECDHE). Unfor- generates a random nonce (the pre-master secret), encrypts it tunately, we discovered that 82.9% of the servers supporting with the server’s public key, and sends it to the server. The DHE used Diffie-Hellman parameters that are weaker than pre-master secret is used by both parties to derive a shared their RSA signature strengths. The resulting sessions are session key for bulk encryption. Since the pre-master secret therefore more vulnerable to a brute-force cryptanalysis attack is encrypted with the server’s public key, only the holder of than if the RSA key exchange (with no forward secrecy) was the server’s private key should be able to decrypt encrypted used. messages. However, the security of the server’s private key We evaluated the throughput of TLS servers and find that is not always as robust as one may wish. An attacker could ECC-based forward secrecy is not much slower, and can even possibly steal private keys from server administrators via be faster, than RSA-based setups with no forward secrecy. social engineering, recover expired private keys from discarded The reason is that with the RSA key exchange, the server must storage devices (that might be less protected), or perform perform an expensive RSA decryption on every key exchange. cryptanalysis with future super computers. In fact, the Heart- With the ECDHE key exchange, the server can RSA-sign its bleed OpenSSL bug [1] makes a point that private keys can be ECDH parameters once and re-use that signature across several silently stolen from servers. Hence, an eavesdropper capturing connections. The server-side online cryptographic operation is encrypted traffic today might be able to decrypt the traffic in then just one elliptic curve multiplication which can be faster the future. A leaked document [2] even suggests that some than a single RSA decryption (of equivalent key strength). governments have surveillance programs to capture backbone Consequently, the traditional performance argument against communications. If a government stores all of the captured forward-secrecy is simply no longer valid given the current traffic and later requests the server’s private key, then past parameter and algorithm choices. Our results suggest that encrypted communications may be decrypted. websites should move to using ECDHE and reap the security and performance benefits. Client Server Lastly, we measured TLS client latencies on thousands of ClientHello −−−−−−! real-world clients using ad networks. We identified a one- ServerHello time performance issue that occurs when a client first sees an Certificate ECDSA signature. Encouragingly, our TLS latency measure- ServerKeyExchange (optional) −−−−−− ments indicate that ECC-based forward secrecy performs no ServerHelloDone ClientKeyExchange worse than plain RSA-based setups on the real-world clients. ChangeCipherSpec Finished −−−−−−! Organization. The rest of this paper is organized as follows. ChangeCipherSpec Section II provides background and related work. Section III −−−−−− Finished presents a survey of TLS servers, which reveals that most DHE servers use insufficient key strengths. Section IV evaluates the Fig. 1: A TLS handshake with no client authentication. The server and client performance of common TLS cipher suites optional ServerKeyExchange message is sent when using that support forward secrecy. Section V discusses the best the Diffie-Hellman key exchange. practices for deploying forward secrecy. Section VI concludes. II. BACKGROUND order. The server also sends its public key certificate over the In this section, we give a brief introduction to the TLS Certificate message to the client. Since the message may be protocol and forward secrecy. We then survey related work. manipulated by a network attacker, the client is responsible of checking whether the server’s certificate is valid (e.g., A. SSL/TLS issued by a trusted certificate authority, matches the server’s Transport Layer Security (TLS) [4], the successor of Secure hostname, not expired nor revoked, etc.). If the certificate is Sockets Layer (SSL) [5], is an encryption protocol designed to accepted, the client generates a pre-master secret, encrypts prevent network adversaries from eavesdropping or tampering this pre-master secret using the server’s public key, and sends sensitive data, while enabling clients and servers to reliably the encrypted secret to the server over a ClientKeyExchange identify each other. TLS allows application-specific protocols message. At this point, both the client and server can compute such as HTTP to be encapsulated and securely transmitted a common master secret (used to derive the session key) from over the underlying transport protocols such as TCP, and is the pre-master secret and the random nonces. Lastly, both increasingly used by web applications, especially by webmail the server and the client send ChangeCipherSpec messages, and online banking websites. indicating that subsequent messages will be encrypted using The TLS protocol is designed to support an extensible set the negotiated cipher suite and session key, starting with an of cipher suites, where each cipher suite defines a combination encrypted Finished message. of authentication, key exchange, bulk encryption, and message B. Forward secrecy authentication code (MAC) algorithms to be used. Forward secrecy, also known as perfect forward secrecy • Authentication algorithms allow communicating parties to (PFS), is an important security property which guarantees that verify the identities of each other, e.g., RSA and DSA. derived session keys cannot be revealed, even if the long- • Key exchange schemes allow peers to securely agree upon term private key is compromised in the future. Especially in on a session key used for bulk encryption, e.g., RSA and the situation where Internet surveillance is a concern, forward Diffie-Hellman. secrecy lets enterprises argue that eavesdroppers simply cannot • Bulk encryption ciphers are used to encrypt the applica- reveal secret data of past communications. However, in TLS, tion data, e.g., AES and RC4. forward secrecy is not necessarily guaranteed. In particular, • Message authentication algorithms are used to generate the RSA key exchange is only secure as long as the server message digests, e.g., SHA-1. can protect its private key. If the server’s private key is ever Recent versions of TLS now support elliptic curve cryptogra- revealed, an attacker can decrypt all recorded sessions by phy (ECC) [6], including ECDSA authentication and Elliptic deriving the pre-master secret using the server’s private key, Curve Diffie-Hellman
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